Sudden cardiac death as a result of QT interval prolongation and subsequent ventricular arrhythmia (torsade de pointes) is a devastating adverse effect of many common medications7, 8. Drug-induced QT prolongation is the number one barrier for new therapeutic agents making it to market. While a number of medications, environmental factors, and genetic factors have been associated with QT prolongation, our ability to predict on an individual basis who will develop QT prolongation, not to mention torsade de pointes, is limited.